Buoyancy effects on objects moving in a bubbling fluidized bed

Abstract

a b s t r a c tThe effect of buoyant forces on the motion of a large object immersed in a bubbling fluidized bed (BFB) was experimentally studied using digital image analysis. The experiments were performed in a 2 D bubbling fluidized bed with glass spheres as bed material and cylindrical objects with different densities and sizes. The object motion was measured using non intrusive tracking techniques. The effect of gas velocity was also analyzed.The circulation of an object in a BFB is defined by several parameters. The object might be able to circulate homogeneously throughout the bed or stay in preferred regions, such as the splash zone or the bottom zone. While circulating, the object moves back and forth between the surface of the bed and the inner regions, performing a series of cycles. Each cycle is composed by sinking and rising paths, which can be one or several, depending on whether a passing bubble is able to lift the object to the surface or the object is detached from it or its drift at an intermediate depth. Therefore, the number of rising paths or number of jumps that the object undergo in a cycle, interleaved with sinking paths, and the maximum attained depth characterize each cycle, together with the mean sinking and rising velocities of the object. In this work, experimental measurements of the probability distributions of the number of jumps and the maximum attained depth, the axial homogeneity of object motion and rising and sinking object velocities are presented for objects with different sizes and densities. The results show a coherent behavior, independent of density and size, for the probability distributions of the number of jumps. This is also true for the maximum attained depth, but only when a proper circulation throughout the bed is ensured. Such a proper circulation and axial homogeneity is, on the other hand, much affected by object density, size and gas velocity. Rising and sinking velocities are highly dependent on gas velocity, as established in well known models of bubble and dense phase velocities. Nevertheless, rising velocities are practically unaffected by object density or size, while sinking velocities show a low dependence on density and a steeper one on size. These results suggest that buoyant forces are relevant during the sinking process, and almost neutral during the rising path.

Simulation of object motion in a bubbling fluidized bed using a Monte Carlo method

“…Variations of the object size for large objects (far larger than the bed material) seems to have some effect on the velocities as it modifies the buoyancy forces over the object (Nienow et al, 1978;Rios et al, 1986;Lim and Agarwal 1994;Rees et al, 2005;Soria Verdugo et al, 2011b). Nevertheless, the effect seems to be feeble for a range of object lengths (Soria Verdugo et al, 2011b), and thus the effect can be neglected for objects that have a proper circula tion in the bed.…”

“…The probability of an object to reach directly the surface (p) was experimentally obtained for a 2D bed by Soria Verdugo et al (2011aVerdugo et al ( , 2011b as previously stated in the Introduction section. The value of p was found to be 0.45 for a wide range of gas velocities and object densities and sizes.…”

“…Nevertheless, the effect seems to be feeble for a range of object lengths (Soria Verdugo et al, 2011b), and thus the effect can be neglected for objects that have a proper circula tion in the bed. In the present work, the results are given for just one object, and no shape, size or density variations are considered.…”

“…The validity of the value of p ¼0.45 for other objects and bed conditions was analyzed in a second work (Soria Verdugo et al, 2011b), varying the dimensionless gas velocity, the bed height, the dense phase diameter and the object characteristics (length and density) and obtaining similar values in all the cases when a proper circulation was observed. When the buoyancy forces are predominant and the circulation is poor (objects remaining in the bed surface or sinking to the distributor and remaining there) the parameter has no longer any meaning.…”

“…The experimental evidence presented in Soria Verdugo et al (2011aVerdugo et al ( , 2011b and concerning the mean sinking and rising velocities and the value of the parameter p provide the basis for the Monte Carlo simulation presented herein.…”